The present invention relates to a method of treatment of ferrous alloy components to improve their friction properties, primarily their resistance to seizing and sticking, without risk of loss of hardness or of deformation.
The invention applies to components in steel or in cast iron with high mechanical properties, i.e. whose tempering temperature is below 200° C.
A person skilled in the art knows that two steel components rubbing against one another, in the absence of lubricant, will seize very quickly. A person skilled in the art also knows that the role of lubricants consists of separating the contacting surfaces by means of a film that promotes sliding and the release of heat. The lubricant film makes it possible to prevent the development of microwelds, which are responsible for seizing and transfer of material.
To be effective, the lubricant film must have a thickness that is greater than the height of the surface asperities. The film thickness largely depends on the physicochemical surface properties and on the surface morphology at the microscopic scale. However, steel straight from machining has surface characteristics such that the thickness of the films of lubricant is as a rule insufficient to ensure continuous lubrication when the loads or speeds become large.
Surface treatments for steels have been developed with the aim of improving either the absorption of the lubricant or the anti-welding characteristics or even both simultaneously.
Two categories of treatment are currently used in the mechanical engineering industry for improving lubricant retention: phosphatation and low-temperature sulphuration. Phosphatation is mainly intended to increase the resistance to seizing of lubricated contacts, and sulphuration additionally endows the surface with properties of inhibition of welding owing to the formation of iron sulphide (hexagonal FeS), and the antiseizing properties are then superior to those obtained with phosphatation.
The physicochemical properties of compounds such as iron phosphate or iron sulphide account for the improved wetting of the lubricants, as the surface energy of these constituents is far higher than that of steel. Furthermore, these constituents have low resistance to shear, as well as excellent capacity for accommodation, which enables them to improve the conditions of running-in and the resistance to wear of contacts that are subjected to surface fatigue.
Electrolytic sulphuration in fused salts in brine is described in FR-A-1 406 530.
Low-temperature sulphuration is effected in a mixture of molten salts at a temperature of about 200° C. using anodic electrolysis, leading to the formation of hexagonal iron sulphide FeS. This last-mentioned method is described in the Applicant's patent FR-A-2 050 754.
Nevertheless, components coated in accordance with the prior art no longer meet the new requirements, especially with respect to the mechanisms employed in the new generations of direct-injection engines.